Start at each end and work towards the center. At the center you will have 2 nail heads showing which are dabbed with tar / caulk. If you want to get really excessive, you can sprinkle some shingle granules over the fresh tar dabs so that passing airplanes and the occasional Godzilla or King Kong looking down can't see the nail heads.

You can go with Houston's method or, you can work from one end to the other. The last 5 to 10 feet you measure the width of shingle exposure and "cheat" them so that you end up with a normal sized shingle at the end.

Even if I were using Houston's method, I would still measure for an even exposure. I do the same thing while laying the shingles up the slope, the last 5 to 10 courses are measured so that I end with full width lap/exposures all the way to the ridge, the ridge being equal to the rest of the slope exposures, rather than having a 2" strip along the ridge.